1. Field of the Invention
The present invention relates to a current driving type transmitter and receiver which can transmit data in the form of differential current through a pair of transmission lines between semiconductor chips in a COG (chip-on-glass) type image display device, and an interface system for COG application which adopts the current driving type transmitter and receiver. More particularly, the present invention relates to a current driving type transmitter and receiver using independent current signals, which can independently generate and transmit positive data current and negative data current for generating differential current indicating data to be transmitted, and an interface system for COG application which adopts the current driving type transmitter and receiver to reduce distortion of transmitted signals.
2. Description of the Related Art
In the conventional art, data transmission between semiconductor chips through transmission lines is implemented in a manner such that a transmitter transmits data in the form of a differential voltage through a pair of differential transmission lines, and a receiver receives data in the form of a voltage, converts the voltage into data to be displayed and utilizes the converted data.
However, such a data transmission method using a voltage difference is likely to be influenced by the length of transmission lines between semiconductor chips. In particular, considering a recent technology trend in which the distance between semiconductor chips is gradually lengthened, the transmission lines have large impedance. Therefore, since the data transmission method using a voltage difference is not appropriate, a data transmission method using current has been proposed in the art.
Concretely speaking, in a conventional mLVDS (mini low voltage differential signaling) scheme using a differential voltage, data and a clock signal are transmitted to a driver IC through a printed circuit board in a multi-drop type. In this case, far end resistors, which are installed at the far end of the printed circuit board, receive the data and the clock signal in the form of voltages. However, in the case of a COG application due to recent trend toward a slim appearance, a timing controller and a driver IC are configured to have a point-to-point drop type, and transmission lines are directly realized on the glass of a panel. Therefore, since a large impedance component exists between the timing controller and the driver IC, distortion of waveforms is likely to seriously occur. In these situations, an LCDS (low current differential signaling) scheme as a data transmission method using current has recently been proposed.
In the current driving scheme as data transmission by current, data is transmitted in the type of current through transmission lines from a transmitter, and a receiver restores the data from the current and utilizes the restored data. The current driving scheme is divided into a single current driving scheme in which one data bit is transmitted through one transmission line, and a differential current driving scheme in which current having different magnitudes is transmitted through two transmission lines and data is restored using a difference in magnitudes of current between the two transmission lines.
In the differential current driving scheme, a transmitter generates bit data using current of different magnitudes and transmits generated bit data through two transmission lines, and a receiver restores the data using a difference in the magnitudes of current between the two transmission lines. In the differential current driving scheme, when compared to the single current driving scheme, while distortion of transmitted signals by noise is not substantial, interference occurs between the transmission lines due to physical positions of the two transmission lines and the parasitic resistance, parasitic inductance and parasitic capacitance of the two transmission lines. As a consequence, the transmitted signals are likely to be distorted, and the time constants of the transmission lines increase, thereby lengthening the transition times of the signals, whereby the transmission speeds of the signals cannot but decrease.
In order to cope with these problems, a differential current driving type transmitter has been disclosed in Korean Patent No. 10-0588752. The differential current driving type transmitter includes a first base current source for supplying first base current Icc1 indicating a base logic state to a true line TX+ of a pair of transmission lines; a second base current source for supplying second base current Icc2 indicating a base logic state to a bar line TX− of the pair of transmission lines; a transition current source for generating transition current Idc indicating a transition logic state as to one of the pair of transmission lines; an equalizing switch for equalizing potentials of the pair of transmission lines; a transition switch for supplying the transition current to the true line or the bar line depending upon a logic value of data to be transmitted; and a transmission controller for controlling switching of the equalizing switch and the transition switch in correspondence to the data to be transmitted.
In the transmitter, one transition current source is connected to one of two base current sources composed of the first base current source and the second base current source depending upon a data signal, and produces a difference in current level between the two transmission lines. Nevertheless, in this case, the first and second base current sources are likely to produce different current values due to deviations in terms of design, process, test condition, etc. As a consequence, the magnitudes of the current applied to the pair of transmission lines by the data signal may vary.
In this regard, referring to FIG. 1, when assuming that the first base current Icc1 supplied by the first base current source is Iref+α, the second base current Icc2 supplied by the second base current source is Iref−α and the transition current Idc supplied by the transition current source is I, current flowing to the true line TX+ of the pair of transmission lines becomes I+Iref+α as the sum of the transition current I and the first base current Iref+α, and current flowing to the bar line TX− is Iref−α. If data transits, current flowing to the true line TX+ of the pair of transmission lines is Iref+α, and current flowing to the bar line TX− becomes I+Iref−α as the sum of the transition current I and the second base current Iref−α. Therefore, every time when transition occurs according to a data value, a problem is caused in that a current value is made unstable.
In the meanwhile, a receiver, including a true line current mirror which is configured to mirror data signal current Irx+ flowing through a true line TX+ of a pair of transmission lines and generate true line mirroring current Irx+, a bar line current mirror which is configured to mirror data signal current Irx− flowing through a bar line TX− of the pair of transmission lines and generate bar line mirroring current Irx−, a true line I-V converter which is configured to generate a true line reception voltage having a level corresponding to the true line mirroring current Irx+, a bar line I-V converter which is configured to generate a bar line reception voltage having a level corresponding to the bar line mirroring current Irx−, and a differential amplifier which is configured to amplify a difference in level between the true line reception voltage and the bar line reception voltage, has been proposed.
Nonetheless, in the receiver, since the I-V converters are respectively provided for the true line and the bar line, errors of the true line and the bar line are respectively converted into voltages and are inputted to the differential amplifier. Due to this fact, a problem is caused in that distortion of the transmitted signals increases. Also, the two converters needed in the receiver increase the size of the receiver, whereby a layout area cannot but be enlarged.